1. Field of the Invention
The present invention relates to a time-division image projector, and more particularly to a time-division image projector that obtains a color projected image by rotating a color wheel, which is a color selection element in which a plurality of color filters are arranged in radiating form.
2. Description of the Related Art
The prior art includes time-division image projectors that, as a basic construction, are provided with a rotatably driven transmission color wheel in which transmission color filters that transmit each of red, green and blue light are arranged in radiating pattern. By transmitting white light that is radiated from a light source through the transmission color wheel, such an image projector isolates and projects each of the colors red, green and blue in time slots.
Japanese Patent Laid-open No. 211358/96 discloses a liquid crystal projector that uses a color filter disk. The technique disclosed in this publication is described as the first example of the prior art.
FIG. 1(a) is a side view showing the construction of time-division image projector 500 of the first example of the prior art, and FIG. 1(b) is an enlarged frontal view of a portion of the prior-art example of FIG. 1(a).
In the time-division image projector 500 shown in FIG. 1(a), white light from white light source 501 is irradiated onto color filter disk 502 shown in FIG. 1(b), whereby red, green and blue light is transmitted in succession synchronized with the rotation of color filter disk 502. The light that is transmitted by color filter disk 502 is condensed by condenser lens 504 and irradiated onto dot matrix liquid crystal display panel 505. Dot matrix liquid crystal display panel 505, synchronized to the timing of the switching of red, green and blue colors of color filter disk 502, displays images corresponding to the red image signal, green image signal, and blue image signal. The thus-formed color image is then projected by way of projection lens 506 onto screen 509.
When this type of color filter disk 502 is used, however, the diameter of color filter disk 502 must be sufficiently greater than the outside diameter of motor 503, with the resulting problem of unwanted bulkiness of the device.
To solve this problem, a method can be considered in which, in place of the color filter disk, a reflective color wheel is used in which reflecting color filters that reflect each of red, green, and blue are arranged in radiating form.
FIG. 2(a) is a side view showing the construction of time-division image projector 600 of the second example of the prior art, and FIG. 2(b) is an enlarged frontal view of a portion of the prior-art example shown in FIG. 2(a).
In time-division image projector 600 shown in FIG. 2(a), white light from white light source 601 is irradiated onto reflective color wheel 602 shown in FIG. 2(b), and red, green, and blue light is successively reflected in synchronism with the rotation of reflective color wheel 602. The light that is reflected by reflective color wheel 602 is condensed by condenser lens 604 and irradiated onto dot matrix liquid crystal display panel 605. Dot matrix liquid crystal display panel 605 displays images corresponding to the red image signal, green image signal, and blue image signal in synchronism with the timing of the switching of red, green and blue of reflective color wheel 602. The color image that is formed in this way is projected onto screen 609 by way of projection lens 606.
A more compact time-division image projector is thus realized through the use of reflective-color wheel 602, which is more compact than color filter disk 502. However, the following problems were encountered in the above-described examples of the prior art.
The first problem is a reduction in the color purity of the projected image.
This problem occurs because light in unwanted wavelength bands tends to be mixed in the projected image because the reflectance of unwanted wavelength bands cannot be adequately suppressed in the characteristic of reflective dichroic filters 602R, 602G, and 602B as shown in FIGS. 3(a)-(c).
A second problem is large irregularities in color in the projected image.
This problem occurs because minute discrepancies inevitably occur in the angles of mounting reflective dichroic filters 602R, 602G, and 602B, with the result that the optical axes of each of the reflected beams fail to accurately match.
A third problem is that, when a rod lens is added between reflective color wheel 602 and condenser lens 604 in the construction of FIG. 2 for the purpose of decreasing irregularities in color over the entire screen of a projected image, the white balance of the projected image is degraded.
This problem occurs because minute discrepancies inevitably occur in the angles in which reflective dichroic filters 602R, 602G, and 602B are mounted, which in turn causes deviations in the axes of the incident light of each of the colors to the rod lens, whereby the projected images of each color fail to accurately match.
It is an object of the present invention to provide a compact time-division image projector in which the projected image has high color purity, few irregularities in color, and a high level of white balance.
To solve the above-described problems, the present invention adopts the following novel and distinctive construction.
The time-division image projector of the present invention features the provision of: light source (1 in FIG. 4) that radiates white light; transmission color wheel (31), in which are mounted a plurality of color filters (31R, 31G, and 31B) that each transmits only light of a specific wavelength band, that selectively transmits only light of specific wavelength bands of the white light from light source (1); reflection element (33), for which the normal of the reflecting surface is arranged at an inclination with respect to light that is irradiated into transmission color wheel (31), that reflects the light of specific wavelength bands that is transmitted by color filters (31R, 31G, and 31B) in a different direction; optical modulation element (41) that is supplied, in time divisions, with a plurality of image signals corresponding to light of each of the specific wavelength bands that has been reflected by reflection element (33), and that modulates, in pixel units, the intensity of light of the specific wavelength bands that has been transmitted by color filters (31R, 31G, and 31B) according to the plurality of image signals; projection lens (5) that projects the light that is emitted from optical modulation element (41); and motor (32) that is linked with axis of rotation (31a) of transmission color wheel (31) and that causes transmission color wheel (31) to rotate in synchronism with the timing of supply of the plurality of image signals that correspond to the light of specific wavelength bands.
In other words, the chief characteristic of the present invention lies in the combined use of transmission color wheel (31) and reflection element (33) to constitute a pseudo-reflective color wheel, thereby combining the advantages of both the transmission and reflective types.
By adopting this construction, the intensity of unwanted wavelength components can be easily reduced through the use of transmission color filters (31R), (31G), and (31B), in which the attenuation rate of the unwanted wavelength bands is characteristically greater than in a reflective color filter.
In addition, the light that is irradiated onto transmission color wheel (31) is transmitted by transmission dichroic filters (31R), (31G), and (31B) and then reflected by a single reflection element (33) that is shared by each of the beams, whereby the optical axes of each of the beams can be made to accurately match. By adopting a means as described hereinabove, the time-division image projector of the present invention exhibits the following effects:
As the first advantage, the projected image has high color purity.
This high color purity results from the use of transmission dichroic filters in the color wheel, which can adequately reduce unwanted wavelength components. As the second advantage, the projected image is free of irregularities in color.
This uniformity of color is achieved not only because a single reflection element can be used in common by each color, but because the reflection element is supported independently of the color wheel or motor, thereby eliminating vibrations in the optical axes of light from the reflection element.
As the third advantage, the white balance of the projected image is not disrupted even when a rod lens is employed.
This maintenance of white balance is achieved not only because a single reflection element can be used in common for each color, but also because the reflection element is supported independently of the color wheel or motor, thereby eliminating vibrations of the optical axis of the light from the reflection element and allowing the optical axis of light irradiated into the rod lens to be kept uniform.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.